The invention relates to a semiconductor device comprising a semiconductor body having a first zone of a first conductivity type adjoining a major surface of the body, a second zone of the second conductivity type which adjoins said major surface and which is bounded within the semiconductor body by the first zone and forms with it a pn junction terminating at said major surface, as well as at least one floating further zone of the second conductivity type having a substantially homogeneous depth. The further zone is located beside the second zone, adjoins said major surface and adjoins the first zone within the semiconductor body, and the distance of the further zone from the second zone is such that, when a sufficiently high reverse voltage is applied across the pn junction, the further zone is located entirely within the depletion zone thus obtained.
A "floating" zone is to be understood, as usual, to mean a semiconductor zone which is not connected to an external current or voltage source.
A semiconductor device of the kind described above is known from the article of Kao and Wolley in Proceedings I.E.E.E., Vol. 55, No. 8, August 1967, p. 1409-1414.
It is known that the breakdown voltage of a planar pn junction is in practice considerably lower than that which could be expected on account of the doping of the said first and second zones forming the pn junction. This is due to local increases of the field strength as a result of surface effects and especially also as a result of the edge curvature of the pn junction. According to the article of Kao and Wolley in Proc. I.E.E.E., these effects are considerably reduced by the presence of one or more floating further zones of the second conductivity type, which in the known device entirely laterally surround the second zone and are located within the range of the depletion zone of the pn junction. Due to these further zones, the reverse voltage across the pn junction is distributed over a considerably larger area, as a result of which the field strength is reduced accordingly.
In the known device, the further zones are provided simultaneously with the second zone and they therefore have the same depth and doping as this second zone. For shallow pn junctions, i.e. for those cases in which the second zone has only a small thickness, not only the edge curvature of the active pn junction, but also that of the pn junctions between the further zones and the first zone is substantial. Consequently, at the areas of the highest edge curvature on the outer side of the pn junctions between the first zone and the further zone or zones, high peaks in the field strength can nevertheless occur, as a result of which the breakdown voltage is reduced. Furthermore, the breakdown voltage depends in a fairly critical manner upon the doping and the mutual spacing of the further zones.